1. Field of the Invention
The invention relates to a device for measuring the momentum transfer spectrum of X-ray quanta elastically scattered in an examination zone, comprising
an X-ray source, PA1 a primary diaphragm device which is arranged between the X-ray source and the examination zone in order to form a primary beam which traverses the examination zone as the surface of a cone and wherefrom the elastically scattered X-ray quanta to be measured emanate, and to form a reference radiation beam which emanates from the X-ray source and traverses the examination zone, PA1 a detector device for detecting X-ray quanta from the examination zone, comprising a plurality of detector elements for measuring elastic scattered radiation from the examination zone and at least one reference detector element on which the reference radiation beam is incident.
2. Discussion of the Related Art
A device as disclosed in EP-OS 556 887 (=U.S. Ser. No. 015096) serves to determine the momentum transfer spectrum of elastically scattered X-ray quanta. The momentum transfer spectra associated with the various layers of the examination zone contain information concerning the crystallographic structure of the material present in these layers. Therefore, they can be used, like a finger print, for the identification of substances present in the examination zone, for example for the inspecting luggage for the presence of, for example explosives.
The attenuation of the X-rays, both before and after the scattering by an object present in the examination zone represents a drawback of this system. It is dependent on the one hand on the energy of the X-rays to be detected and on the other hand on the type and distribution of attenuating material in the examination zone. In order to enable correction for such attenuation, the similarly attenuated reference radiation beam is detected by the reference detector element.
The reference radiation in the known device is produced by a pencil beam whose position in space is given by the connecting line between the focus of the X-ray source and the center of the detector device. It is a drawback that the intensity of the pencil beam is substantially higher than the intensity of the elastically scattered X-rays measured by the detector device. In order to prevent saturation of the reference detector element by the reference radiation during a measurement, either the cross-section of the pencil beam must be very small (only a few square micrometers) or the pencil beam must be substantially attenuated by an absorber, the effect of the attenuation on low-energy X-ray quanta then being stronger than that on high-energy X-ray quanta. It is another drawback of the known device that the pencil beam traverses a part of the examination zone other than that traversed by the conical primary radiation beam and the X-ray quanta elastically scattered therein. If the material is not sufficiently homogeneously distributed across the examination zone, errors will thus occur in the attenuation correction.